Acetyl-CoA is one of significantly important intermediates in metabolic pathways of microorganisms. Various metabolites are produced via acetyl-CoA. Well-known examples of such substances produced via acetyl-CoA include amino acids such as L-glutamic acid, L-glutamine, L-proline, L-arginine, L-leucine, and L-isoleucine; organic acids such as acetic acid, propionic acid, butyric acid, caproic acid, citric acid, 3-hydroxybutyric acid, 3-hydroxyisobutyric acid, 3-aminoisobutyric acid, 2-hydroxyisobutyric acid, methacrylic acid, and poly-3-hydroxybutyric acid; alcohols such as isopropyl alcohol, ethanol, and butanol; acetone; and polyglutamic acid.
In most microorganisms, acetyl-CoA is produced using a sugar such as glucose as a carbon source. The sugar is first converted into pyruvate via a metabolic pathway called the glycolytic pathway, such as the Embden-Meyerhof pathway, the Entner-Doudoroff pathway, or the pentose phosphate pathway. Subsequently, pyruvate is converted into acetyl-CoA by the action of the enzymes pyruvate decarboxylase, pyruvate formate-lyase, and the like. In this process, since carbon dioxide (CO2) and formate are generated as byproducts, not all of the carbons derived from the sugar are fixed as acetyl-CoA. Therefore, several studies have been carried out with the aim of achieving re-fixation of CO2 in order to increase the yield of acetyl-CoA.
In microorganisms, there are several known pathways for fixing carbon dioxide as a carbon source (Applied and Environmental Microbiology, 2011; 77(6), 1925-1936). Specific examples of the pathways include the Calvin-Benson cycle, the reductive TCA cycle, the Wood-Ljungdahl pathway, the 3-hydroxypropionate cycle, and the 4-hydroxybutyrate cycle. The Calvin-Benson cycle is a CO2 fixation pathway existing in plants and photosynthetic bacteria, and comprising about 12 enzymes. In the Calvin-Benson cycle, CO2 is fixed by ribulose-1,5-bisphosphate carboxylase (RubisCO) and, ultimately, glyceraldehyde 3-phosphate is produced. The reductive TCA cycle is found in microaerophilic bacteria and anaerobic bacteria including green sulfur bacteria, and comprises 11 enzymes. The reductive TCA cycle comprises the CO2 fixation enzymes acetyl-CoA carboxylase and 2-oxoglutarate synthase which requires ferredoxin as a coenzyme. In the reductive TCA cycle, pyruvate is produced from CO2 by the reverse reaction of the usual TCA cycle. The Wood-Ljungdahl pathway is found in anaerobic microorganisms such as acetic acid-producing bacteria, and comprises 9 enzymes. In the Wood-Ljungdahl pathway, CO2 and formate bound to a coenzyme are reduced by formate dehydrogenase, CO dehydrogenase, etc., and, ultimately converted into acetyl-CoA. The 3-hydroxypropionate cycle is found in Chloroflexus bacteria and the like, and comprises 13 enzymes. In the 3-hydroxypropionate cycle, CO2 is fixed by the action of acetyl-CoA (propionyl-CoA) carboxylase, and acetyl-CoA is produced via malonyl-CoA and the like. The 4-hydroxybutyrate cycle exists in archaebacteria and the like. In the 4-hydroxybutyrate cycle, CO2 is fixed by the actions of pyruvate synthase, acetyl-CoA (propionyl-CoA) carboxylase, and phosphoenolpyruvate carboxylase, whereby acetyl-CoA is produced via 4-hydroxybutyryl CoA and the like.
Several approaches of producing a useful substance by introducing a carbon dioxide fixation pathway to a microorganism that produces a useful compound have been proposed. For example, International Publication Nos. WO 2009/094485 and WO 2010/071697 disclose approaches for producing acetyl-CoA from CO2 by using a microorganism to which a pathway similar to the Wood-Ljungdahl pathway of acetic acid bacteria has been introduced. WO 2009/046929 discloses an approach for producing lactic acid from CO2 by using a microorganism to which hydrogenase and tetrahydrofolate lyase have been introduced. WO 2011/099006 proposes a cycle in which CO2 is fixed via a carbon dioxide fixation reaction into acetyl-CoA or a malonyl-CoA reduction reaction. German Patent Application Laid-open No. 102007059248 proposes production of acetyl-CoA via a pathway similar to the 4-hydroxybutyrate cycle.